The present invention relates to electrochemical cells. An electrochemical cell is comprised of two or more half-cells. In most, but not all electrochemical cells, each half-cell is separated by a semi-permeable membrane. For a simple electrochemical cell, the two half-cell compartments are synonymous with the anode and cathode. For any two half-cell compartment electrochemical cell to be able to provide electrical energy, the Gibbs free energy for the overall reaction has to be negative, and the electrochemical reactions, at both the anode and cathode, have to occur at an appreciable rate to be practicable.
In order for the electrochemical cell to provide electrical energy, an outside path, commonly referred to as a circuit, must be provided through which electrons may flow. A completed circuit, in which electrons flow from the anode to the cathode through the circuit, is obtained by interfacing an electrical conductor of an uninterrupted circuit at contacts on both the anode and cathode. These contact points are commonly referred to as terminals of the anode electrode, and cathode electrode, respectively. Concomitantly, ions contained in the electrolyte solution flow between the anode and cathode to maintain a balance of charge. In the invention claimed within, the electrical energy provided is significantly augmented in both voltage potential and capacity in accordance with Hückel's rule for aromaticity for the cathode material.
Hückel's rule identifies aromatically stabilized compounds by using the number of π electrons in an all carbon cyclic compound with alternating double and single bonds (such systems are referred to as annulenes). If the π electron count, in an annulene, equals 4n+2 where n is zero or any positive integer, then the compound is aromatic. The opposite of aromatically stabilized compounds are known as antiaromatic compounds. Hückel's rule predicts antiaromatic compounds will occur when the number of π electrons equals 4n where n is zero or any positive integer. Antiaromatic annulenes have higher energies (and hence, instabilities) than do aromatic compounds; this is true in both experiment and in theory. This antiaromatic instability often causes a distortion in the cyclic molecule (annulene) resulting in non-planarity.
The addition of a single electron to an antiaromatic annulene (4n π-electron system) results in an approach to aromaticity, although just one electron will not technically lead all the way to aromaticity. Thus, adding or removing even one electron to some cyclic compounds can have a dramatic effect upon the stability and shape of that annulene. Likewise, adding, or removing even one electron from a 4n+2 π electron system can also have a marked effect upon the stability and shape of that molecule.
As a consequence of the 4n+2 π-electron rule, the cyclooctatetraene (“COT”) moiety, more fully outlined below, which contains 4n π-electrons, is a very strong electron acceptor. COT (or [8]annulene) is an eight membered ring moiety that is herein being exploited as the basis for the proposed cathode systems. The COT moiety readily accepts the first electron, going from eight to nine π-electrons, with a voltage potential of approximately 1.8 V (in HMPA vs. Li/Li+). The very large potential is due to the approach to aromatic character. This voltage varies somewhat dependent upon the solvent/electrolyte system. A second electron is accepted with a voltage potential of approximately 1.46 V (in HMPA vs. Li/Li+), where upon the annulene becomes aromatic with 4n+2 π-electrons.
The neutral (4n π-electron) COT moiety is distorted from planarity and exists in a tub conformation. In this way the COT moiety avoids planar anti-aromatic character. The addition of a single electron forces the COT system into a planar (D4h) conformation. The second electron forces the now fully aromatic annulene into a planar D8h conformation.
It is important to note that this system has a very high capacity for accepting electrons, since the COT moiety can reversibly accept two electrons per unit molecule. In terms of specific capacity (capacity per unit mass), COT reacting with lithium to yield COT-Li2 represents a theoretical capacity of approximately 450 A-hrs/kg. This capacity far exceeds other reversible lithium (and lithium ion) cathode technologies. The theoretical specific energy is simply the product of the specific capacity and average voltage (here, it is taken to be 1.63 V), such that the COT-Li2 system represents approximately 740 W-hrs/kg. Moreover, by accepting two electrons, the COT moiety and substituted COT moieties can achieve enormous thermodynamic stability and the attributes of aromatic systems. This high stability of COT-Li2 (the COT moiety here has 10 π-electrons) can be demonstrated by its persistence at elevated temperatures, which can exceed 400° C.
For the foregoing reasons there is a need for an electrochemical cell which incorporates a new cathode material, in which redox (oxidation-reduction) couples rapidly and reversibly exchange multiple electrons in a narrow potential range to provide the electrochemical cell with high voltage, high energy, and high power.